Nowadays, presensitized plates (PS plates) and other plates in which positive-type photosensitive materials mainly composed of a diazo dye and a phenolic resin or negative-type photosensitive materials mainly comprising an acrylic monomer or prepolymer are used are in practical use in platemaking for lithographic offset printing. However, such materials are invariably low in sensitivity and therefore platemaking is performed employing contact exposure through film negatives on which images have been preliminarily recorded. On the other hand, recent advances in the technologies of computer-assisted image processing, mass storage of data and data transmission have made it possible to computerize the operations of input of originals, revision, editing, layout and pagination throughout and put into practical use electronic editing systems which can instantly output data on terminal plotters in remote places through high-speed communication or satellite communication networks. In particular, in the field of newspaper printing which requires promptness, electronic editing systems are highly demanded. In the fields where originals are currently stored in the form of films and printing plates are prepared by copying based thereon as demanded, the spread of mass storage media such as optical disks will perhaps lead, in one aspect, to storage of originals as digital data in such recording media.
However, few direct-type plates for platemaking capable of giving printing plates directly from the terminal plotter output have been put into practical use. Accordingly, even where an electronic editing system is operated, it is still a current practice that output data are received on silver salt photographic films and printing plates are prepared by contact exposure of PS plates therethrough. One reason is that direct-type printing plate precursors having sufficiently high sensitivity for making printing plates within a practical time period using the output plotter light source (e.g. He-Ne laser, semiconductor laser) are difficult to develop.
Electrophotographic photosensitive materials are thought to have such a high photosensitivity as to render them capable of providing direct-type printing plates.
As another method of prepafing printing plates using the technique of electrophotography, a process is already known which comprises forming toner images and then removing the nonimage areas of the photoconductive layer. For example, mention may be made of those electrophotographic printing plate precursors that are described in JP-B-37-17162, JP-B-38-6961, JP-B-38-7758, JP-B-41-2426 and JP-B-46-39405 (the term "JP-B" as used herein means an "examined Japanese patent publication") and JP-A-50-19509, JP-A-50-19510, JP-A-52-2437, JP-A-54-145538, JP-A-54-134632, JP-A-55-105254, JP-A-55-153948, JP-A-55-161250, JP-A-57 147656 and JP-A-57-161863 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), among others.
In the above process, it is necessary to remove the nonimage areas of the electrophotographic photosensitive material by etching for exposing the hydrophilic surface and, therefore, the binder resin is often a binder resin capable of leaving said surface as a result of dissolution or swelling in an alkaline solvent.
However, in printing using the printing plates obtained in the above manner, in particular in newspaper printing on roll-form paper using a rotary press, printing-due staining readily occur on the prints in the regions corresponding to the end portions of the printing plate, although no problem is produced in cases where the paper sheets to be printed do not include such end portion regions, as in printing paper sheets smaller in size than the printing plate using an ordinary sheet press. Such staining is particularly remarkable when development is performed with a toner in the manner of reversal development.
For preventing end region staining due to the lithographic printing plates obtained from electrophotographic plates for lithographic platemaking by reversal development, it has been proposed that an insulating resin layer should be provided on the cut end sides (end faces) of said electrophotographic plates (JP-A-63-178240). This proposal is based on the thought that one cause of printing staining due to lithographic printing plates obtained from electrophotographic plates for lithographic plate by reversal development in the manner mentioned above should be the unnecessary toner adhesion to the cut end faces of the electrophotographic plates in the step of reversal development, which adhesion allows ink to adhere to those portions as well, leading to staining and that application of an insulating resin to the end faces of said electrophotographic plates might prevent the toner from adhering to said portions during reversal development. Other measures have been proposed, as described in JP-A-2-61654 and JP-A-2-66566.
On the other hand, photosensitive plates for lithographic platemaking (photosensitive lithographic printing plate precursor) whose support is an aluminum plate are commercially available as PS (presensitized) plates and are in wide use.
Printing with the printing plates prepared from PS plates by imagewise exposure, development and other processings encounters the same problem. Ink adhering to the end portions is also transferred to paper, causing staining and thereby seriously impairing the commercial value of the prints.
One known method of preventing such staining of the end portions of printing plates comprises rounding off the angles from the end portions of the aluminum support by-means of a file or knife, as disclosed in JP-B-57-46754. This method has a drawback in that the printing plates should be rounded off one by one or, in other words, said method is not suited for large quantity processing.
Furthermore, JP-A-59-97146 proposes a method suited for mass production which comprises treating the end faces of photosensitive plates for lithographic platemaking for desensitization. The desensitizing composition is mainly composed of a hydrophilic resin and a strongly acidic compound. This method, however, can solve the end portion staining only to an unsatisfactory extent.
The present inventors found that even when such measures as mentioned above are taken, printing-due end region staining still occurs when lithographic printing plates obtained by toner image formation by reversal development and the subsequent removal of the nonimage areas of the photoconductive layer are used in printing newspapers and so on. The present inventors investigated the causes thereof and, as a result, found that when an insulating layer is provided, as mentioned above, substantially no toner adheres to the end faces but the insulating resin layer itself remains on the end faces and allows ink adhesion, hence printing staining. In other words, it was found that while the insulating resin layer formed on the end faces indeed prevents toner adhesion to the end faces in the step of reversal development, it itself has oleophilic property (namely ink receptivity) and therefore, if it remains, it allows ink adhesion to the end faces, thus failing to prevent printing staining.
Furthermore, the advent of PS plates free from end portion staining and suited for mass production is desired.